Two mystery relatives in military uniforms

Two mystery relatives in military uniforms

I'm trying to find out a bit about my paternal grandmother's family, and unfortunately she and my father both passed many years ago.

I have two photos that were hers and am trying to see if I can figure anything out based on the uniforms being worn in them. One is her uncle -- I only know this because he signed the photo for her -- and he's wearing a Navy officer's uniform of some sort. I have no information on this uncle, but she was born in 1910, so I guess he probably would have been born anywhere from the 1870's to 1890's.

The other photo is a man whose relation to my grandmother is an utter mystery, though I have seen a couple of other photos of her with a man who might be the same guy. He's wearing some sort of military uniform, someone has written "Ossifer" on the photo, and the photo looks pretty old, but that's all I've got.

Any ideas anyone might have about what branches of the service these uniforms are from or what time period they might be from would be much-appreciated clues. Thank you!